Vehicular emissions which are the principle pollutants that have negative effects on public health and the natural environment are generally recognised to be carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx) and particulate matter.
Several solutions have been proposed to remove and/or control these principle pollutants, some of which focus on the engine design and some of which focus on control of emissions from exhaust systems.
Typical exhaust systems that exist to remove and/or control such emissions comprise a NOx Trap, an oxidation catalyst to catalyse conversion of CO and HC, which are present as a result of incomplete combustion of the fuel in the engine, and a filter substrate to remove particulate matter.
Lean NOx traps (LNT) (also known as NOx adsorber catalyst); such as disclosed in U.S. Pat. No. 5,473,887, use a trap, for example an alkaline earth metal oxide that adsorbs NOx during the lean mode of operation. Exhaust gas is typically rich in NO which is converted to NO2 over a platinum group metal-containing oxidising catalyst, such as platinum or ruthenium, and the NO2 is trapped and stored on the alkaline earth metal oxide, such as barium carbonate, which is incorporated within the platinum group containing catalyst. The NOx is then released from the barium under rich conditions, when the oxygen concentration in the exhaust emission is decreased, and reduced with a suitable reductant, for example diesel fuel for diesel engines, using a rhodium catalyst as a promoter. The rhodium catalyst may be incorporated on the platinum group containing catalyst or it may be located downstream of the LNT.
One mechanism commonly given for NOx trapping from a lean exhaust gas for this formulation is:NO+0.5O2→NO2  (1);BaO+NO2+0.5O2→Ba(NO3)2  (2),wherein in reaction (1), the NO reacts with oxygen on active oxidation sites on the platinum group metal catalyst to form NO2. Reaction (2) involves adsorption of the NO2 by the storage material in the form of an inorganic nitrate.
At lower oxygen concentrations and/or at elevated temperatures, the nitrate species become thermodynamically unstable and decompose, producing NO or NO2 according to reaction (3) below. In the presence of a suitable reductant, these nitrogen oxides are subsequently reduced by carbon monoxide, hydrogen and hydrocarbons to N2, which can take place over the reduction catalyst (see reaction (4)).Ba(NO3)2→BaO+2NO+1.5O2 or Ba(NO3)2→BaO+2NO2+0.5O2  (3);andNO+CO→0.5N2+CO2 (and other reactions)  (4).
In the reactions of (1)-(4) above, the reactive barium species is given as the oxide.
However, it is understood that in the presence of air or lean engine exhaust gas most of the barium is in the form of the carbonate or possibly the hydroxide. The skilled person can adapt the above reaction schemes accordingly for species of barium other than the oxide.
A problem in the use of LNT, e.g. in both diesel and gasoline applications, is that the engine fuel also contains sulphur, and this is converted to sulphur dioxide (SO2) during fuel combustion. SO2 is oxidised to sulphur trioxide (SO3) by the oxidation catalyst component of the LNT and the SO3 is adsorbed on the NOx adsorber by a similar mechanism to that of NO2. There are a finite number of active sites on the NOx trapping component for adsorbing the NOx and so the presence of sulphate on the NOx trapping component reduces the capacity of the NOx trapping component as a whole to adsorb NOx. Therefore, in order to retain sufficient NOx trapping capability, sulphur must be periodically removed from the LNT. However, sulphates of NOx trapping components such as barium are more stable than nitrates in lean exhaust gas and generally higher temperatures and/or richer conditions for longer periods are required to remove SOx than for desorbing NOx.
Desulphation can be accomplished by a variety of techniques including by a series of short, rich pulses. A significant problem with desulphating a LNT using richer than normal exhaust gas compositions is that the sulphate is removed as hydrogen sulphide. This compound has a characteristic and unpleasant rotten egg odour, 0.0047 ppm is the recognition threshold, the concentration at which 50% of humans can detect the characteristic odour of hydrogen sulphide, and accordingly it is desirable to prevent/limit its emission to atmosphere.
This problem has been reduced to a certain extent in recent years for diesel fuels because Ultra Low Sulphur Diesel (fuel of maximum sulphur content of 15 ppm (wt.)) is now available in the US. In Europe, diesel fuel with a maximum sulphur limit of 10 ppm has been available since the beginning of 2010. As a result, LNT desulphation procedures are required less often. However the extremely low odour threshold of hydrogen sulphide means that even a small emission to atmosphere is undesirable.
Particulate filters have been shown to be extremely effective at removal of particulate matter over the entire particle size range. However, these filters have limited capacity for trapping particulate matter before the pressure drop becomes excessive. Therefore it is necessary to periodically regenerate the particulate filter. Passive regeneration may not readily take place as combustion of the retained particulate matter in the presence of oxygen requires higher temperatures than those typically provided by engine exhaust, particularly in the case of diesel passenger car exhausts. One effective method to lower the combustion temperature of the trapped particulate matter on the particulate filter is the addition of a catalysed washcoat to the filter substrate. Compositions of catalysed washcoats used are similar to those used in oxidation catalysts and typically comprise at least one platinum group metal supported on a suitable support material. Suitable support materials include alumina, silica-alumina, ceria or a mixed oxide or composite oxide of ceria and zirconia, and mixtures thereof. Such filters are typically known as catalysed soot filters (CSF).
A diesel exhaust system comprising a LNT and a downstream CSF is known. For example SAE 2001-01-2065 entitled “Cummins Light Truck Diesel Engine Progress Report” discloses such a system. It is acknowledged by the authors in this report that there was no consideration of the effects of deterioration or contamination due to sulphur poisoning and no attempt was made to desulphate during any of the driving cycles.
WO2008/075111 discloses an apparatus comprising a lean burn internal combustion engine which comprises an exhaust system for treating a flowing exhaust gas from the engine having a LNT, a CSF, a means for enriching the exhaust gas to provide an enriched exhaust gas intermittently during operation in order to remove sulphate that is adsorbed on the LNT and a compound located downstream of at least some of the LNT which is effective in removing and/or converting hydrogen sulphide that is produced during the sulphate removal process. The hydrogen sulphide removal and/or converting material is selected from the group consisting of oxides of nickel, calcium, iron and barium. The hydrogen sulphide removal and/or converting material may be located in a variety of positions in the exhaust system, which include between the LNT and CSF, on the CSF, between the CSF and the exhaust system exit. The only discussion about why the hydrogen sulphide removal and/or converting material is placed at the chosen position is with respect to any platinum group metal oxidation catalysts as nickel oxide can poison the hydrocarbon and carbon monoxide activity of the platinum group metal catalyst.
U.S. Pat. No. 5,196,390 discloses a method of suppressing hydrogen sulphide formation by a three way catalyst by incorporating one or more of oxides of nickel, iron and manganese into an undercoat layer disposed on a monolith substrate. A topcoat overlying the undercoat comprises standard three way catalyst material. This structure avoids any interaction between the three way catalyst and the hydrogen sulphide suppressing material.
Applicant's PCT application no. PCT/GB2012/051407 filed 19 Jun. 2012 discloses an exhaust system for internal combustion engines and a catalysed substrate for use in an exhaust system. The exhaust system comprises a lean NOx trap and the catalysed substrate. The catalysed substrate has a first zone and a second zone, wherein the first zone comprises a platinum group metal loaded on a support and the second zone comprises copper or iron loaded on a zeolite. The first zone or second zone additionally comprises a base metal oxide or a base metal loaded on an inorganic oxide. Also provided are methods for treating an exhaust gas from an internal combustion engine using the exhaust system. The exhaust system is capable of storing NH3 generated in a rich purge, reacting the NH3 with slip NOx from the NOx trap, controlling H2S released from NOx trap desulphation, and oxidizing slip hydrocarbons and carbon monoxide. When the catalysed substrate is a filter substrate, it is also capable of removing soot from exhaust system.
The different materials that are available to remove and/or convert the hydrogen sulphide each operate most efficiently over a defined temperature range. For example oxides of iron are known to be effective at lower temperatures, typically 400-800° C., and oxides of manganese are known to be effective at higher temperatures, which in some cases can be in excess of 1000° C. Mixing together some of these different materials may not be enough to extend the temperature range for removal and/or conversion of the hydrogen sulphide as there may be negative interactions based on such combinations.
We have now identified a system which is capable of treating both NOx and particulate matter emitted from the exhaust system of an internal combustion engine whilst also removing and/or converting unwanted hydrogen sulphide emissions over a broad temperature range. This is achieved by providing a catalysed substrate monolith downstream of an LNT, whose desuiphation process leads to the formation of undesirable hydrogen sulphide, wherein separate zones within the catalysed substrate comprise different materials for removal and/or conversion of hydrogen sulphide emissions. Particulate matter can be trapped on a separate filter substrate or more preferably the catalysed substrate monolith is a catalytic soot fitter.
Accordingly, in a first aspect, the invention provides a zoned catalysed substrate monolith comprising a first zone and a second zone, wherein the first zone and the second zone are arranged axially in series, wherein the first zone comprises a first base metal oxide or a first base metal loaded on an inorganic oxide and the second zone comprises a second base metal oxide or a second base metal loaded on an inorganic oxide wherein the second base metal is different from the first base metal.
In a preferred embodiment, neither the first zone nor the second zone nor (where present) the third zone contains copper or iron loaded on a zeolite.
According to a second aspect, the invention provides an exhaust system for an internal combustion engine comprising:
(a) a lean NO trap; and
(b) a zoned catalysed substrate monolith according to any preceding claim, wherein the first zone is oriented to the upstream side,
wherein the zoned catalysed substrate monolith is disposed downstream from the lean NOx trap.
The lean NOx trap typically includes a NOx adsorbent for the storage/trapping of NOx and an oxidation/reduction catalyst. The oxidation/reduction catalyst generally comprises one or more noble metals, preferably platinum, palladium, and/or rhodium. Typically, platinum is included to perform the oxidation function and rhodium is included to perform the reduction function. The rhodium catalyst may be positioned downstream of the LNT.